PHOTOELECTRON SPECTROSCOPY (PES) · PHOTOELECTRON SPECTROSCOPY (PES) Law of Photoelectric effect...
Transcript of PHOTOELECTRON SPECTROSCOPY (PES) · PHOTOELECTRON SPECTROSCOPY (PES) Law of Photoelectric effect...
PHOTOELECTRON SPECTROSCOPY (PES)
Law of Photoelectric effectAlbert Einstein, Nobel Prize 1921
Kaiser-Wilhelm-Institut (now Max-Planck-Institut) für Physik Berlin, Germany
INTRODUCTION
High-resolution electron spectroscopyKai Siegbahn, Nobel Prize 1981
Uppsala university, Sweden
Valence band
EB
ωhE
EF
Evac
Detector (Ef)
φ
Ek
Discrete core levels
≈
kinB EE −−= φωh
ωhBE
kinE
φ
Where, = Binding energy referred to the Fermi level= Photon energy= Work function of spectrometer
= Kinetic energy of ejected photoelectron
BASIC CONCEPT
ENERGY REFERENCE
Same level
e-
Fermi level
Spectrometer
Vacuum level
φspec.
.speckinB EE φω −−= h
sampleBkin EE φω −−= h
)( . samplespecsampleBkin EE φφφω −−−−= h
.specBkin EE φω −−= h
Core level
φsample
Fermi level
Vacuum level
EB
Sample
ωh
BASIC REQUIREMENT OF PES SYSTEM
1. A monochromatic photon source.
2. An electron energy analyser (which can disperse the emittedelectrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy.
3. A high vacuum environment.
LIGHT SOURCES
X-rays
Al K α 1486 eV
Mg Kα 1254 eV
XPS, ESCA
He-lamp 21.2 eV40.8 eV
Ne-lamp 16.8 eV26.9 eV
UV
H-lamp 4 ≤ hv ≤ 12 eV
UPS
Synchrotron radiation PES
SYNCHROTRON RADIATION
•Unique feature of Synchrotron radiation�Ultra-bright light, high polarization�Highly directional, collimated beam�Wide spectrum of wavelength, stretching from IR to X-rays
•Spot size ~1x3 mm2
•High energy resolution
HardX-ray
Ultravioletlight
Visiblelight
Infraredlight
Radiowave
Wavelength (meter)
Atom nucleus
Atom Protein Cell
10 eV4000 eV
BASIC REQUIREMENT OF PES SYSTEM
1. A monochromatic photon source.
2. An electron energy analyser (which can disperse the emittedelectrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy.
3. A high vacuum environment.
ELECTRON ENERGY ANALYSERS
ELECTRON ENERGY ANALYSER
Ek
Retarding Voltage (Eret.)
Epass
Ek-Eret=Epass
ENERGY RESOLUTION
Schematic view of a spherical deflector analyser. Source and exit slit widths s and w. Sphere radii R1 and R2 and the optical radius Rm = (R1+R2)/2.
Vf
E
N(Ek)
EkinE
IN
N(Ek)
EkinE
∆E
OUT
V f
Epass
ENERGY RESOLUTION
)(2
1
1
2
R
R
R
R
e
EV pass
f −=
2
22α++=∆
mmpass R
w
R
s
E
E
broading a giveAnalyser Q
Energy resolution
Choose:s, w and Epass� small
BASIC REQUIREMENT OF PES SYSTEM
1. A monochromatic photon source.
2. An electron energy analyser (which can disperse the emittedelectrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy.
3. A high vacuum environment.
WHY ULTRA HIGH VACUUM (UHV) IS NEEDED ?
Consider an ideal gas.The number of gas atoms/molecules striking a unit area of a surface per second is given by;
T
P
R
N
V
NRT
N
NnRTpVBut
M
RTwhere
V
NI
a
a
n
=⇒==
==π
νν 8
4
)(1
105.3)(1
106.28
4 222
2224 torrinp
smMT
px
m
Ninp
smMT
px
MT
pR
R
NI a
n
≈
==π
Ex. Assume an atom of radius of ca. 3 Å. Then there is room for ca. 1015
atom on a cm2.
With p ≈ 10-6 torrM = 28kg/kmol In ≈ 3x1014 (1/cm2s)T = 300 K
If every atom sticks to the surface a monolayer is formed in about 3 sec!.
At p ≈ 10-10 torr a monolayer is formed in about 3x104 seconds≈ 8 hours.Big differences (in sticking probability) between different materials.
(From experience at 10-10 torr Si, Au etc days, Al, Cu etc. hours, Ti, Mo etc. minutes)
AN EXAMPLE
HOW UHV CAN BE OBTAINED?
•Good choice of material selected ex. stainless steel
•Good design of chamber
•Good pumping systems
•Long time baking
On a larger scale, the sample surface is hit with photon flux focused in a small spot. Electrons emitted by the photon are from ca. 10-100Å of the surface layers depending on electron escape depth in the sample investigated.
SAMPLE SIZE?
Electrons : give a high surface sensitivitySinceλ = mean free path is small in solids
λ = average distance an electron travels before it is inelastically scattered, i.e. Suffers energy losses
λ(Ek) increases monotonically with Ekin (for Ekin ≥ 50 eV)
SENSITIVITY
The flux of electrons of a given energy and momentum decaysexponentially with the distance travelled in a material.
I0I
x
θ
θcos
x
θλ cos0
⋅−
=x
eII
λx
eII−
= 0
Small λ gives high surface sensitivity
Electrons that have not beeninelastically scattered have beengenerated close to the surface.
The surface sensitivity can be enhanced by selecting a larger electronemission angle, θ. Normal emission , θ=0o.
ELECTRON’S FLUX
A SIMPLE LAYER ATENUATION MODEL
d d d
Io Io Io
θ
θλ
λ
λλλ
cos
0
0
)3
()2
()(
0
1
1
1
1
.....)1(
dtot
dtot
ddd
tot
e
II
e
II
eeeII
−
−
−−−
−=
−=
=++++=
1cos
cos
0
−=
=
=
−
θλ
θλ
d
bulk
Surf
d
totbulk
Surf
eI
I
eII
IId d d
Io Io Io
θ
d
Surf layer
Normal emission
Angle dependent
z 0d
. 1 ,
overlayer from )1(
overlayer no withi.e.
0
0
dthicknessofoverlayerwitheII
Thus
toponoverlayerwhenebyattenuatedissignalsubstrateThe
ecdzceI
cdzceI
d
Subst
Overl
d
dd
z
Overlayer
zCleanSubstrate
−=
−=∝
=∝
−
−−
∞ −
∫
∫
λ
λ
λλ
λ
λ
λ
Photoelectron Spectroscopy (PES) + Angle resolved photoelectron spectroscopy (ARPES)
Examine core-level hv ≈ 100-4000 eV
•Elementals annalysis
•Quantitative analysis
Thickness
Location
Examine valence levels hv ≈ 10-50 eV
•Surface band structure
•Density of state (DOS)
UNIQUE ADVANTAGE FOR PES TECHNIQUES
KINETIC ENERGY (eV)
ELEMENTALS ANALYSIS
Binding Energy 2s = 117eV Binding Energy 2p = 73 eV
Binding Energy 2s = 149 eV Binding Energy 2p = 99 eV
EFB exhibits chemical shifts (~1-10 eV)
depending on chemical surrounding�Information about the surrounding elements can be obtained.
In solids a specific signal from the atoms in the outermost surface layers can be obtainedsince they feel a different surrounding(potential) than atoms in the bulk. Surface shiftes < 1 eV.
Chemical shifts
Surface shifts
KINETIC ENERGY (eV)
QUANTITATIVE ANALYSIS
4f
5d
5p
5s
PHOTOIONIZATION CROSS SECTIONS
Graphitelike carbon
C 1s SiC
286 284 282Binding Energy (eV)
θe=75o
θe < 75o
Int e
nsit y
(re l
ativ
e un
its)
CHARACTERIZATION
SiO2
SiC
Carbon ?
)(
)(;
)(
)( 2
SiCI
SiOISi
SiCI
gIC ==
Emission angle0 20 40 60
80
Si
C0
2
4
Rat
ios
C/Si x 20
SiO2
C=0.05 Å
25.5 Å
SiC
)(
)(;
)(
)( 2
SiCI
SiOISi
SiCI
gIC ==
)1(
)1(
cos
coscos
2 −⋅=
−⋅⋅=
eSi
ox
eC
g
eC
ox
d
SiSiC
SiSiO
dd
CSiC
Cg
eC
CSi
eeC
CC
θλ
θλθλ
+
⋅⋅⋅⋅= 1ln
AA
BB
B
A
C
C
I
IOxide
σσλ
+
⋅
⋅⋅⋅⋅=
−
1ln1
λ
σσλ
d
CC
BB
B
C eC
C
I
IGraphite
Thickness
Layers attenuation model
A:=oxide, B:= bulk SiC , C:= graphite
TECHNIQUES
ADVANTAGES
•Good energy resolution (chemical shifts & surface shifts)•Semiquantitative analysis, within a few %, fairly easy
DISADVANTAGE
•Poor spatial resolution (in most cases).
PHOTOELECTRON SPECTROSCOPY